Composite materials are used in ever increasing quantities in a wide variety of industries. For example, commercial aircraft incorporate composite materials in primary and secondary structure due to the favorable mechanical properties of composite materials. Such favorable properties may translate into reduced weight of the aircraft and increased payload capacity and fuel efficiency. Furthermore, composite materials may provide an extended service life for the aircraft as compared to aircraft formed of conventional metallic construction.
Rework of composite structures is occasionally required in order to remove inconsistencies in the composite structure. Such inconsistencies may comprise localized areas that may not conform to product and/or performance specifications for any one of a variety of reasons. For example, areas of the structure may have out-of-tolerance inconsistencies which may include, without limitation, voids, dents, delaminations or porosity in the composite structure. Such localized areas of composite structures may be reworked in order to reduce or eliminate the inconsistencies. The reworking process may include removing material from the structure and replacing the removed material with a composite patch that may be bonded to the structure. The patch may be formed as a stack of plies of composite material which may be of the same or different type of material from which the composite structure is formed.
Prior to installing the patch in the rework area for bonding, it is typically desirable to remove excess air and/or volatiles that may be trapped within the plies of the composite material that make up the patch. In addition, it may be desirable to consolidate or compact the composite plies. One method of removing such volatiles is by performing a debulking operation wherein a vacuum debulking assembly may be applied over the patch. The debulking operation may be performed at a location away from the rework area. The vacuum debulking assembly may include heating the patch to facilitate the removal of volatiles and improve the compaction of the patch.
Following debulking, the heated patch may then be transferred to the rework area of a composite structure containing the rework area such as on an aircraft. The patch may be applied to the rework area in the desired orientation and may be cured in place with an adhesive to allow the patch to conform and bond to the rework area. A second option may be to temporarily mount the patch in the rework area without adhesive and then apply heat to partially cure the patch and allow the patch to conform to the rework area. The partially cured patch may then be removed from the rework area and fully cured at a location away from the rework area such as in an autoclave followed by re-installing and permanently bonding the patch to the rework area with adhesive.
For certain composite materials such as those with a bismaleimide (BMI) matrix material, the required temperatures for processing the composite materials are relatively high. For example, debulking operations performed on BMI materials may require temperatures in excess of 250° F. Curing of BMI materials may require a temperature of approximately 440° F. Furthermore, the length of time for performing a debulking operation on a patch formed of BMI materials may be relatively short due to the relatively fast rate at which BMI materials cure. In this regard, BMI composite materials at elevated temperatures may become brittle in a relatively short period of time.
The combination of the relatively high temperatures at which BMI materials must be processed and the relatively short processing window presents challenges in transferring a BMI patch to the rework area following debulking Furthermore, due to its relatively low thermal mass, the patch may cool at a relatively rapid rate during the transfer process. The cooling rate may occur at a faster rate at the edges of the patch. Such non-uniform cooling of the patch may induce thermal stresses in the patch that may lead to undesirable results. Such thermal stresses may increase when the relatively high temperature patch is installed in a relatively cooler rework area.
In addition, the high temperatures required for processing certain composite materials presents challenges associated with handling the patch when the patch is transferred to the rework area. For example, failure to support the entirety of the patch during the transfer process may result in sagging of unsupported portions of the patch such as at the edges which may affect fit-up of the patch to the rework area. In addition, sagging of unsupported areas of the patch may affect the integrity of the patch following final cure.
As can be seen, there exists a need in the art for a system and method for processing a composite patch wherein the temperature of the patch may be maintained. Furthermore, there exists a need in the art for a system and method for processing a patch that facilitates the transfer of the patch from a vacuum debulking operation to the rework area in a manner wherein the patch is supported. Preferably, such system is simple in construction and low in cost.